CONCLUSIONS FRO^r EXPERIMENTS 187 



PERIODICITY IN ORGANISMS IN RELATION T(J THE AGE CYCLE 



Before leaving the question of the nature of senescence and 

 rejuvenescence it is necessary to call attention to their relation to 

 other periodic or cyclical changes in the organisms. According 

 to the conception developed here, there is nothing unique in the 

 processes of senescence and rejuvenescence; they are, on the con- 

 trary, of the same general character as many other changes in rate 

 of metaboUsm in the organism, the chief difference being that the 

 factors concerned in the age changes are the more stable and less 

 rapidly changing features of the substratum, while other shorter 

 cycles may result from changes in less stable features. In fact, 

 it is not possible to make any sharp distinction between the age 

 changes and many other periodicities. The differences are differ- 

 ences of degree rather than of kind. Recognition of this fact is 

 important, because senescence has often been regarded as a rather 

 mysterious process, quite different from anything else in the life 

 cycle, but the experimental evidence points to a very different 

 conclusion. 



The more or less regularly periodic or cyclical changes are among 

 the most conspicuous and characteristic features of living organisms. 

 They range in the individual from momentary, evanescent changes, 

 such as occur in stimulation and the return to the original condition 

 which follows, to the changes of the age cycle which often coincide 

 with the whole Hfe of the individual. Some of these periodic changes 

 are of course directly determined by external conditions, such as 

 temperature, light, etc., while, as regards others, internal factors 

 are more important. Any extended consideration of these various 

 periodicities is quite beyond the present purpose, but the fact that 

 many of them seem to be more or less similar in character to the 

 age cycle, except as regards the time factor, demands some sort of 

 interpretation. According to the physico-chemical conception of 

 the organism, many different periodic changes in rate of metabolism 

 are possible, for different conditions in the substratum which accel- 

 erate or retard the rate of metabolism may arise and disappear with 

 very different rapidity, and the variety of more or less dehnitely 

 periodic phenomena in life is in full agreement wilii theoretical 

 possibility. 



